Silver halide crystal particles are sensitized by visible rays, ultraviolet rays or other radiation rays to form latent images, which are converted into visible images by development, and, thus, there are various kinds of silver halide photographic materials at present. Usable typical silver halides are silver iodide, silver bromide, silver chloride and mixtures of crystals thereof. The selection of the kind of the silver halide depends upon the use of the photographic materials and the necessary characteristics thereof. For instance, silver iodobromide particles having a relatively large particle size are used in photographic materials for camera films, which generally require a high sensitivity; and silver idodbromide or silver chlorobromide particles having a small particle size are used in photographic materials for duplication or printing, which have relatively low sensitivity. The kind of the silver halide, the shape of the crystal form thereof and the size of the particles are important for the determination of the characteristics of the silver halide emulsion. The matter is described in detail in T. H. James, The Theory of the Photographic Process, 4th Ed. (Macmillan Co., Ltd. in N.Y.); and C. Hasse, H. Frieser and E. Klein, Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden (Akademische Verlagsgesellschaft, Frankfurt an Main, 1968).
In order to provide silver halide photographic materials of high quality, the necessary factors of the silver halide emulsion are to be properly selected in view of the use of the materials and the necessary characteristics thereof, as described above. However, the selection is not always satisfied with ease. For example, it is said that the photographic materials for prints have relatively low sensitivity, but the recent photographic materials are being required to have high sensitivity, since the printing step and the development step are becoming faster and the time for the exposure is being shortened more and more. The most fundamental means for intensifying the sensitivity of the silver halide emulsions is to enlarge the particle size of the silver halide particles so as to increase the light absorbance per one particle. In the case where the emulsion is color-sensitized, an electron is imparted to the silver halide and the light absorbance of the sensitizer dye which will be indispensable for the formation of latent images may be increased. However, these means are not always satisfactory. Moreover, the enlargement of the particle size often results in the interruption of the acceleration of the development process. The color sensitization will often cause development retardation or desilvering retardation, and, in general, the increment of the amount of the color sensitizer as added is hardly effective for the increment of the sensitivity of the photographic materials. Accordingly, it is considered extremely important to elevate the sensitivity of the silver halide particles without enlargement of the particle size thereof. The silver halides are generally chemically sensitized prior to the actual use thereof, and various chemical sensitization means are known, including, in general, a method using a sulfur sensitizer such as sodium thiosulfate, a method using a gold sensitizer such as potassium chloroaurate, a method involving the formation of reduced and sensitized nuclei and a method comprising a combination of these means.
These conventional means have various problems. In particular, the silver halide particles having the (111) surface are difficultly sensitized by sulfur sensitization as compared to those having (100) surface, and the sensitization efficiency is low when the gold sensitization is not used therewith. Japanese Patent Application (OPI) No. 30748/83 describes an effective sulfur sensitization method to be carried out in the presence of a silver halide solvent (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). This method is, however, defective in that the fog is not sufficiently lowered at all when silver chlorobromide is used and in that the photographic characteristic is apt to vary quite often after the chemical sensitization. Japanese Patent Application (OPI) No. 125612/83 describes another sulfur sensitization method as characterized by the two-stage constitution of pAg and/or temperature in the sensitization, and this says that the sulfur sensitization for a high silver chloride emulsion is effective. It is to be noted, however, that this method is to merely achieve a general sulfur sensitization level by a general technical means. Also, this method cannot be expected to increase further the sensitivity of the photographic materials. Under the circumstances, any other effective sulfur sensitization is required which is capable of attaining a sufficient increment of the sensitivity of the photographic materials.